1. Oxygenates blood.
2. Removes Carbon dioxide from the body.
   

What happens in the lungs when we breath?

The respiratory system works using the principle of diffusion. Gases (e.g. oxygen and carbon dioxide) move from areas of high concentration, across pressure gradients, into areas of lower pressure. Due to the fact that thoracic cavity volume is changed using the respiratory muscles (internal and external intercostal muscles, abdominals and diaphragm) the air flows in and out of the lungs depending on the external pressure.

When we inhale (breathe in) the respiratory muscles make the chest cavity bigger creating lower pressure in the lungs than outside the body. Air then flows in to equalise the pressure.

When we exhale (breathe out) the respiratory muscles make the chest cavity smaller and raise the pressure of air in the lungs. At the first opportunity this air will flow out of the lungs.

Diffusion of Gases in the Lungs

Due to a pressure gradient in the alveoli diffusion of gases takes place. High levels of oxygen in the alveoli causes the gas to diffuse into the red blood cells through the alveoli wall where oxygen pressure is low in the deoxygenated blood as it flows past.

As this is happening the high levels of carbon dioxide in the deoxygenated blood diffuse into the low carbon dioxide pressure area of the alveoli. Blood that leaves the alveoli is now oxygenated and it heads back to the heart via the pulmonary vein.